The activated sludge process is used routinely for biological treatment of municipal and industrial waste water. The basic activated sludge process consists of three components, i.e. a reactor in which the microorganisms are kept in suspension and aerated (aeration tank), a liquid-solids separation  (clarifier), and a recycle system for returning sludge removed from the clarifier back to the reactor.

Most Activated Sludge system are configured as secondary waste water treatment units when they receive waste water that have been pretreated by primary sedimentation. Primary sedimentation is most efficient at removing settleable solids, whereas the biological processes are essential for removing soluble, colloidal, and  particulate (suspended) organic substances.